The platinum group metals or PGMs (platinum, palladium, rhodium, iridium, osmium, and ruthenium) are becoming increasingly important to the global economy. Platinum is used for jewelry and chemical catalysts, and blends of platinum, palladium, and rhodium are used in catalytic converters.
PGMs frequently occur together in naturally occurring sulfide minerals along with base metals, such as nickel, copper, and iron. By way of illustration, a typical PGM sulfide ore contains from about 0.01 to about 0.3 oz/ton platinum, from about 0.01 to about 0.3 oz/ton palladium, from about 0.01 to about 0.1 oz/ton rhodium, and also contains variable amounts of nickel, copper, and iron.
FIG. 1 depicts a typical process for recovering platinum and palladium and base metals from a mined material. The feed 100 is comminuted 104, the comminuted material floated 108 to form a flotation concentrate (containing PGMs and base metals as sulfides), the concentrate melted 112 to form a furnace matte (containing the PGMs and base metals as sulfides) and a stag (containing the silicate, chromite, and other gangue minerals), the matte converted 116 to remove some sulfur and most of the iron and form a converter matte (containing the PGMs and copper and nickel base metals), the converter matte comminuted 120 to form a comminuted converter matte, the comminuted converter matte atmospheric leached 124 with sulfuric acid to form a first pregnant leach solution 128 (containing most of the nickel, and iron as ferrous) and a first leached material or residue 132 (containing most of the PGMs, some nickel, and most of the copper), the first leached material 132 separated 136 from the first pregnant leach solution 128, the first leached material 132 autoclave leached 140 using sulfuric acid to form a second pregnant leach solution 144 (containing the balance of the nickel, the copper, and sulfuric acid) and a second leached material or residue 148 (containing the PGMs and iron oxides), and the second leached material 148 separated 152 from the second pregnant leach solution 144. A nickel product 156 is recovered 160 from the first pregnant leach solution 128, a copper product 164 is recovered 168 from the second pregnant leach solution 144, and the PGM concentrate 172 containing platinum, palladium and rhodium is recovered 180 from the second leached material 148.
A significant problem with this circuit is the disposal of the sulfur in the sulfide minerals. Although a significant portion of the sulfur is removed in the converting step 116 (as sulfur dioxide gas), the sulfur contained in the matte which feeds the base metal refinery must be removed from the circuit to control sulfur levels in the various steps and maintain a sulfur balance within the process.
Typically, sulfur levels are controlled by removing sulfur in the nickel recovery step 160. One approach to removing sulfur is to crystallize the nickel as nickel sulfate as a nickel product 156. Crystallization of the nickel as nickel sulfate, however, significantly reduces the value of the nickel product 156. Yet another approach is to recover the nickel by electrowinning. Electrowinning of the nickel produces sulfuric acid at the anode. A bleed stream is removed from the nickel (and/or copper) electrowinning circuits, and the sulfuric acid in the bleed stream is disposed of as a sulfate by neutralizing the sulfuric acid with a base. Sodium hydroxide is commonly used as the base. Sodium hydroxide is expensive and produces a product, namely sodium sulfate, which is difficult to dispose of and has a resale value that is much less than that of the sodium hydroxide from which it is formed. Calcium hydroxide, or oxide, or carbonate, another possible base, is much less expensive than sodium hydroxide but produces calcium sulfate or gypsum. Gypsum levels in the solution are difficult to control, and gypsum can precipitate in the circuit due to changes in temperature and ionic strength, thereby creating operational problems, especially within the nickel electrowinning circuit. Moreover, neutralizing the bleed stream with calcium hydroxide can increase metal losses (and decrease metal recoveries) as the metal hydroxide precipitates can be difficult to separate from the calcium sulfate precipitates.